Method for producing a monocrystalline layer of lithium niobate by transferring a seed layer of yttria-stabilized zirconia to a silicon carrier substrate and epitaxially growing the monocrystalline layer of lithium niobate and substrate for epitaxial growth of a monocrystalline layer of lithium niobate

ABSTRACT

A process for producing a monocrystalline layer of LNO material comprises the transfer of a monocrystalline seed layer of YSZ material to a carrier substrate of silicon material followed by epitaxial growth of the monocrystalline layer of LNO material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/IB2019/000200, filed Mar. 26, 2019,designating the United States of America and published as InternationalPatent Publication WO 2019/186263 A1 on Oct. 3, 2019, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1800256, filed Mar. 28, 2018.

TECHNICAL FIELD

The present disclosure relates to a process for producing amonocrystalline layer of lithium niobate (LNO) material and a substratefor the epitaxial growth of such a monocrystalline layer of LNOmaterial.

BACKGROUND

Certain materials are not currently available as a monocrystallinesubstrate in the form of a large-diameter wafer. Moreover, certainmaterials may be available in large diameter but not with certaincharacteristics or specifications in terms of quality, in particular,with regard to the density of defects or the required electrical oroptical properties.

BRIEF SUMMARY

The present disclosure aims to overcome these limitations of the priorart by providing a process for producing a monocrystalline layer of LNOmaterial and a substrate for the epitaxial growth of such amonocrystalline layer of LNO material. In this way it is possible toaddress the problem of size of the monocrystalline substrates of LNOmaterial currently available.

The present disclosure relates to a process for producing amonocrystalline layer of LNO material comprising the transfer of amonocrystalline seed layer of yttria-stabilized zirconia (YSZ) materialto a carrier substrate of silicon material followed by epitaxial growthof the monocrystalline layer of LNO material.

In advantageous embodiments, the monocrystalline seed layer has athickness of less than 10 μm, preferably less than 2 μm, and morepreferably less than 0.2 μm.

In advantageous embodiments, the transfer of the monocrystalline seedlayer of YSZ material to the carrier substrate of silicon materialcomprises a step of joining a monocrystalline substrate of YSZ materialto the carrier substrate followed by a step of thinning of themonocrystalline substrate of YSZ material.

In advantageous embodiments, the thinning step comprises the formationof a weakened zone delimiting a portion of the monocrystalline substrateof YSZ material intended to be transferred to the carrier substrate ofsilicon material.

In advantageous embodiments, the formation of the weakened zone isobtained by implanting atomic and/or ionic species.

In advantageous embodiments, the thinning step comprises detaching atthe weakened zone so as to transfer the portion of the monocrystallinesubstrate of YSZ material to the carrier substrate of silicon material,in particular, the detaching comprises the application of thermal and/ormechanical stress.

In advantageous embodiments, the joining step is a molecular adhesionstep.

In advantageous embodiments, the monocrystalline seed layer of YSZmaterial is in the form of a plurality of tiles each transferred to thecarrier substrate of silicon material.

In advantageous embodiments, the carrier substrate of silicon materialcomprises a detachable interface configured to be detached by means of alaser debonding technique and/or chemical attack and/or by means ofmechanical stress.

The present disclosure also relates to a substrate for epitaxial growthof a monocrystalline layer of LNO material, characterized in that itcomprises a monocrystalline seed layer of YSZ material on a carriersubstrate of silicon material.

In advantageous embodiments, the monocrystalline seed layer of YSZmaterial is in the form of a plurality of tiles.

In advantageous embodiments, the carrier substrate of silicon materialcomprises a detachable interface configured to be detached by means of alaser debonding technique and/or chemical attack and/or by means ofmechanical stress.

The present disclosure also relates to a process for producing amonocrystalline layer of Li_(x)K_(y)Na_(z)Ti₁Nb_(m)O₃ material having alattice parameter close to that of the LNO material comprising thetransfer of a monocrystalline seed layer of YSZ material to a carriersubstrate of silicon material followed by epitaxial growth of themonocrystalline layer of Li_(x)K_(y)Na_(z)Ti₁Nb_(m)O₃ material.

The present disclosure also relates to a process for producing amonocrystalline layer of Li_(x)K_(y)Na_(z)Ti₁Nb_(m)O₃ material having alattice parameter close to that of the LNO material comprising thetransfer of a monocrystalline seed layer of SrTiO₃ or CeO₂ or MgO orAl₂O₃ material to a carrier substrate of silicon, sapphire, Ni or Cumaterial, followed by epitaxial growth of the monocrystalline layer ofLi_(x)K_(y)Na_(z)Ti₁Nb_(m)O₃ material.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present disclosure will be betterunderstood from reading the following detailed description withreference to the appended drawings, wherein:

FIG. 1 illustrates a process for producing a monocrystalline layer ofLNO material according to one embodiment of the present disclosure and asubstrate for the epitaxial growth of such a monocrystalline layer ofLNO material according to this embodiment of the present disclosure;

FIG. 2 illustrates a process for producing a monocrystalline layer ofLNO material according to another embodiment of the present disclosureand a substrate for the epitaxial growth of such a monocrystalline layerof LNO material according to this other embodiment of the presentdisclosure;

FIG. 3 illustrates a process for producing a monocrystalline layer ofLNO material according to yet another embodiment of the presentdisclosure and a substrate for the epitaxial growth of such amonocrystalline layer of LNO material according to this other embodimentof the present disclosure;

FIG. 4 illustrates a process for producing a monocrystalline layer ofLNO material according to yet another embodiment of the presentdisclosure and a substrate for the epitaxial growth of such amonocrystalline layer of LNO material according to this other embodimentof the present disclosure;

FIG. 5 illustrates a process for producing a monocrystalline layer ofLNO material according to yet another embodiment of the presentdisclosure and a substrate for the epitaxial growth of such amonocrystalline layer of LNO material according to this other embodimentof the present disclosure.

To improve the readability of the figures, the various layers are notnecessarily shown to scale.

DETAILED DESCRIPTION

FIG. 1 illustrates a carrier substrate 100 of silicon material to whicha monocrystalline seed layer 200 of YSZ material is transferred. Othermaterials for the monocrystalline seed layer 200 may be envisaged suchas SrTiO₃, CeO₂, MgO or Al₂O₃, these having a lattice parameter close tothat of the LNO material. The carrier substrate 100 of silicon materialmay also be replaced with a carrier substrate 100 of sapphire, Ni or Cumaterial. The use of silicon has the advantage of opening up the fieldof application of layers of LNO material not only for 300 mm-typelarge-scale equipment, but also making it compatible with themicroelectronics industry, for which the requirements in terms ofacceptance on the production line for exotic material other thansilicon, especially LNO, are high. The step of joining 1′ themonocrystalline seed layer 200 of YSZ material to the carrier substrate100 of silicon material is preferably carried out by way of a molecularadhesion step. This molecular adhesion step comprises a bonding step,preferably at ambient temperature, and is followed by an anneal forconsolidating the bonding interface, which is usually carried out athigh temperatures of up to 900° C. or even 1100° C. for a duration of afew minutes to a few hours. Regarding a carrier substrate of sapphirematerial, the step of joining 1′ the monocrystalline seed layer to thecarrier substrate is also preferably carried out by way of a molecularadhesion step using typical conditions of the same order of magnitude asmentioned above. Regarding a carrier substrate of Ni or Cu material, thestep of joining 1′ the monocrystalline seed layer to the carriersubstrate is replaced by a step of depositing the Ni or Cu material onthe monocrystalline seed layer, for example, via deposition byelectrodeposition (ECD) or electroforming (electroplating). Thistechnique usually includes the use of a tie layer and stripping and isknown per se and will not be described in more detail here.

FIG. 1 schematically shows the step of joining 1′ a monocrystallinesubstrate 20 of YSZ material to the carrier substrate 100 of siliconmaterial. It follows a step of thinning 2′ the monocrystalline substrate20 of YSZ material after having been joined to the carrier substrate 100of silicon material. FIG. 1 schematically shows the thinning step 2′,which may be implemented, for example, by means of chemical and/ormechanical etching (polishing, grinding, milling, etc.). Thus, themonocrystalline seed layer 200 of YSZ material may be obtained, whichwill serve as the monocrystalline seed for a step of epitaxially growing3′ the monocrystalline layer 300 of LNO material on the substrate forepitaxial growth of a monocrystalline layer of LNO material 10 shownschematically in FIG. 1 . Those skilled in the art would be capable ofadjusting the parameters used for epitaxially growing a monocrystallinelayer of LNO material usually used in homoepitaxy or heteroepitaxy on abulk monocrystalline substrate in order to optimize the step ofepitaxially growing 3′ the monocrystalline layer 300 of LNO material onthe substrate for epitaxial growth of a monocrystalline layer of LNOmaterial 10 of the present disclosure. Epitaxy of the LNO materialtherefore takes place by way of MOCVD at typical temperatures of between650 and 850° C. using precursors known to those skilled in the art.Incidentally, the present disclosure is not limited to epitaxy of theLNO material but extends to certain composites of trigonal crystalstructure of Li_(x)K_(y)Na_(z)Ti₁Nb_(m)O₃ type having a latticeparameter close to that of the LNO material.

It should be noted that the thermal expansion coefficient of the carriersubstrate 100 dominates the thermal behavior of the substrate forepitaxial growth of a monocrystalline layer of LNO material 10 duringthe step of epitaxially growing 3′ the monocrystalline layer 300 of LNOmaterial. This is due to the low thickness, preferably less than 1 μm,of the monocrystalline seed layer 200 of YSZ material relative to thetotal thickness of the substrate for epitaxial growth of amonocrystalline layer of LNO material 10, which is of the order ofseveral tens to hundreds of μm. Incidentally, the YSZ material is chosenso as to provide a monocrystalline seed layer having a lattice parameterthat is as close as possible to the lattice parameter chosen for themonocrystalline layer 300 of LNO material, preferably the latticeparameter in the relaxed state in order to allow epitaxial growthresulting in as few defects as possible in the monocrystalline layer 300of LNO material. Incidentally, the material of the carrier substrate 100advantageously has a thermal expansion coefficient, which isparticularly close to the thermal expansion coefficient of the LNOmaterial for the same reasons of decreasing defects in themonocrystalline layer 300 obtained by epitaxy. Preferably, a carriersubstrate 100 of sapphire material would therefore be used for thepresent disclosure.

FIG. 2 schematically shows one embodiment of the process for producing amonocrystalline layer of LNO material, which differs from the embodimentdescribed in conjunction with FIG. 1 in that the monocrystallinesubstrate 20′ of YSZ material undergoes a step of implanting 0″ atomicand/or ionic species in order to form a weakened zone delimiting aportion 200′ of the monocrystalline substrate 20′ of YSZ materialintended to be transferred to the carrier substrate 100′ of siliconmaterial, and in that the thinning step 2″ comprises detaching at thisweakened zone so as to transfer the portion 200′ of the monocrystallinesubstrate 20′ of YSZ material to the carrier substrate 100′ of siliconmaterial, in particular, this detaching comprises the application of athermal and/or mechanical stress. The advantage of this embodiment isthus to be able to recover the remaining part 201 of the startingmonocrystalline substrate 20′ of YSZ material, which may thus be usedagain to undergo the same process again and thus decrease costs. Thesubstrate for epitaxial growth of a monocrystalline layer of LNOmaterial 10′ thus illustrated in FIG. 2 is used for the step of growing3″ the monocrystalline layer 300′ of LNO material as already describedin the process described in conjunction with FIG. 1 . In general, theimplantation step 0″ takes place using hydrogen ions. One advantageousalternative well known to those skilled in the art consists in replacingall or some of the hydrogen ions with helium ions. A hydrogenimplantation dose will typically be between 6×10¹⁶ cm⁻² and 1×10¹⁷ cm⁻².The implantation energy will typically be between 50 to 170 keV. Thus,the detaching typically takes place at temperatures of between 300 and600° C. Thicknesses of the monocrystalline seed layer of the order of200 nm to 1.5 μm are thus obtained. Immediately after the detachingoperation, additional technological steps are advantageously added withthe aim of either strengthening the bonding interface or of restoring agood level of roughness, or of correcting any defects, which may havebeen be generated in the implantation step or else to prepare thesurface of the seed layer for the resumption of epitaxy. These stepsare, for example, polishing, (wet or dry) chemical etching, annealing,chemical cleaning. They may be used alone or in a combination, whichthose skilled in the art will be capable of adjusting.

FIG. 3 differs from the embodiments described in conjunction with FIG. 1and FIG. 2 in that the substrate for epitaxial growth of amonocrystalline layer of LNO material (10, 10′) comprises a detachableinterface 40′ that is configured to be detached. In the case of acarrier substrate 100 of silicon material, it may be a rough surface,for example, of silicon material, joined with the monocrystalline seedlayer during the joining step. Else, a rough interface may be presentwithin the carrier substrate 100 of silicon material, the latter, forexample, obtained by joining two silicon wafers. Another embodimentwould be to introduce, at the face to be joined with the monocrystallineseed layer, a porous silicon layer that is liable to split during theapplication of a mechanical and/or thermal stress, for example, byinserting a blade at the edge of the wafer known to those skilled in theart or by applying an anneal. Obviously, this interface is chosen so asto withstand the other mechanical and/or thermal stresses experiencedduring the process of the present disclosure (e.g., detaching, epitaxialgrowth, etc.). In the case of a carrier substrate of sapphire material,it may be a stack of silicon oxide, silicon nitride and silicon oxide(what is called an ONO-type structure) produced on the face of thesapphire to be joined with the monocrystalline seed layer. Such a stackis liable to undergo detachment at the silicon nitride layer whenapplying a laser that passes through the sapphire carrier substrate(detaching or debonding of the “laser lift-off” type). Those skilled inthe art will be capable of identifying other processes for producingthis detachable interface. These various detaching configurations thusmake it possible either to transfer the epitaxial layer to a finalcarrier, which is not compatible with the growth parameters or toprepare a thick film of LNO material of freestanding type.

FIG. 4 schematically shows one embodiment of the process for producing amonocrystalline layer of LNO material, which differs from theembodiments described in conjunction with FIGS. 1-3 , wherein themonocrystalline seed layer 2000′ of YSZ material is in the form of aplurality of tiles (2001′, 2002′, 2003′) each transferred to the carriersubstrate 100″ of silicon material. The various tiles may take any shape(square, hexagonal, strips, etc.) and have different sizes varying froma few mm² to several cm². The spacing between the chips may also varysignificantly depending on whether what is sought is a maximum densityof coverage (in this case a spacing of less than 0.2 mm will preferablybe chosen) or conversely a maximum spread of the tiles within thesubstrate (in this case the spacing may be several millimeters and evencentimeters). For each tile, a person skilled in the art would becapable of applying their desired transfer and they are not limited to aparticular process. Thus, it is possible to envisage applying thetechnical teachings described in conjunction with the processillustrated schematically in FIG. 1 or the technical teachings describedin conjunction with the process illustrated schematically in FIG. 2 , oreven a combination of the two. Thus, it is possible to join 1′″monocrystalline substrates (2001, 2002, 2003) of YSZ material, whichhave a size smaller than the size of the carrier substrate 100″ in orderto create by thinning 2′″ on this latter the monocrystalline seed layers(2001′, 2002′, 2003′) for the epitaxial growth 3′″ of a monocrystallinelayer (3001, 3002, 3003) of LNO material on each tile of the substratefor epitaxial growth of a monocrystalline layer of LNO material 10″.

The various embodiments described in conjunction with FIGS. 1 to 4 thusopen up the possibility of co-integration of components made in themonocrystalline layer of LNO material with components made in thecarrier substrate of silicon material. This latter may simply be asilicon substrate, but it may also be an SOI-type substrate comprising asilicon oxide layer separating a silicon substrate from a thin siliconlayer. In the case of the embodiments described in conjunction withFIGS. 1 to 4 , access to the carrier substrate may be achieved simply byway of lithography and etching known to those skilled in the art. In thecase of the embodiment described in conjunction with FIG. 4 , it is alsopossible just to choose the locations of the tiles and their spacing.

FIG. 5 schematically shows one embodiment, which differs from theembodiment described in conjunction with FIG. 4 in that the carriersubstrate 100″ and subsequently the substrate for epitaxial growth of amonocrystalline layer of LNO material 10″ comprises a detachableinterface 40 that is configured to be detached, for example, by means ofa laser debonding (laser lift-off) technique and/or chemical attackand/or by means of mechanical stress. This would allow part of thecarrier substrate 100″ to be removed as already mentioned in conjunctionwith FIG. 3 . One example would be the use of a carrier substrate 100 ofSOI type comprising a silicon oxide layer separating a silicon substratefrom a thin silicon layer. This oxide layer could be used as adetachable interface 40 by selective etching this oxide layer, forexample, by immersion in a hydrofluoric (HF) acid bath. This option ofdetaching a buried layer by means of chemical etch is particularlyadvantageous when it is in combination with treating a plurality ofsmall substrates. Specifically, the range of under-etches is generallylimited to a few centimeters or even a few millimeters if it is desiredto retain industrially reasonable treatment conditions and times.Treating a plurality of small substrates allows the initiation ofseveral chemical etching fronts by virtue of possible access to theburied layer between each tile, rather than just at the extreme edges ofthe substrates, which may be up to 300 mm in diameter. In the case of anSOI-type carrier substrate, it is thus possible to partially remove thethin layer of silicon between the tiles in order to allow the initiationof several etching fronts.

Since the thin layer of silicon has a predetermined thickness (which mayvary between 5 nm to 600 nm, or even thicker depending on the intendedapplication), it could thus be used to form microelectronic componentsand thus allow the co-integration of components based on LNO materialsin a single substrate.

Thus, after having formed the monocrystalline layer (3001, 3002, 3003)by epitaxy, it is also possible to conceive joining this structure to afinal substrate and detaching, at the detachable interface 40, a part ofthe carrier substrate 100″. The final substrate may thus provideadditional functionalities, which are, for example, incompatible withparameters of the growth carried out previously (for example, finalsubstrate of flexible plastic type or final substrate comprising metallines). Additionally and in general, the detachable interface is notnecessarily located inside the carrier substrate but may also be locatedat the interface with the seed layer of YSZ material joined to thiscarrier substrate (for example, a stack of a layer of silicon nitridebetween two layers of silicon oxide allows laser debonding, particularlysuitable for a carrier substrate of sapphire type) as already describedin conjunction with FIG. 3 .

The invention claimed is:
 1. A process for producing a monocrystallinelayer of lithium niobate (LNO) material, comprising: joining two siliconwafers to form a carrier substrate of silicon defining a detachableinterface within the carrier substrate of silicon, the detachableinterface including a joint between the two silicon wafers including onesurface of the two silicon wafers that is roughened; transferring amonocrystalline seed layer of yttria-stabilized zirconia (YSZ) materialdirectly onto the carrier substrate of silicon including joining amonocrystalline substrate of YSZ material to the carrier substrate ofsilicon; and epitaxially growing the monocrystalline layer of LNOmaterial on the monocrystalline seed layer of YSZ material, wherein thejoining the monocrystalline substrate of YSZ material to the carriersubstrate of silicon comprises molecular adhesion of the monocrystallinesubstrate of YSZ material to the carrier substrate of silicon, themolecular adhesion including bonding the monocrystalline substrate ofYSZ material to the carrier substrate of silicon at ambient temperatureand annealing a bonding interface between the monocrystalline substrateof YSZ material and the carrier substrate of silicon to consolidate thebonding interface.
 2. The process of claim 1, wherein themonocrystalline seed layer has a thickness of less than 10 μm.
 3. Theprocess of claim 2, wherein the joining the monocrystalline substrate ofYSZ material to the carrier substrate is followed by thinning themonocrystalline substrate of YSZ material.
 4. The process of claim 3,wherein the thinning comprises forming a weakened zone delimiting aportion of the monocrystalline substrate of YSZ material to betransferred to the carrier substrate of silicon.
 5. The process of claim4, wherein the formation of the weakened zone comprises implantingatomic and/or ionic species into the monocrystalline substrate of YSZmaterial.
 6. The process of claim 4, wherein the thinning comprisesdetaching at the weakened zone so as to transfer the portion of themonocrystalline substrate of YSZ material to the carrier substrate ofsilicon.
 7. The process of claim 3, wherein the monocrystalline seedlayer of YSZ material is in the form of a plurality of tiles eachtransferred to the carrier substrate of silicon.
 8. The process of claim3, wherein the detachable interface is configured to be detached by alaser debonding technique and/or chemical attack and/or by applicationof mechanical stress.
 9. The process of claim 1, wherein themonocrystalline seed layer has a thickness of less than 2 μm.
 10. Theprocess of claim 1, wherein the monocrystalline seed layer has athickness of less than 0.2 μm.
 11. The process of claim 1, wherein thetransfer of the monocrystalline seed layer of YSZ material to thecarrier substrate of silicon comprises joining a monocrystallinesubstrate of YSZ material to the carrier substrate, followed by thinningthe monocrystalline substrate of YSZ material.
 12. The process of claim6, wherein detaching at the weakened zone so as to transfer the portionof the monocrystalline substrate of YSZ material to the carriersubstrate of silicon comprises application of a thermal and/ormechanical stress to the monocrystalline substrate of YSZ material. 13.The process of claim 1, wherein the monocrystalline seed layer of YSZmaterial is in the form of a plurality of tiles each transferred to thecarrier substrate of silicon.
 14. The process of claim 1, wherein thedetachable interface is configured to be detached by a laser debondingtechnique and/or chemical attack and/or by application of mechanicalstress.
 15. The process of claim 1, wherein the annealing is performedat a high temperature up to 1100° C.
 16. A substrate for epitaxialgrowth of a monocrystalline layer of lithium niobate (LNO) material,comprising; a monocrystalline seed layer of yttria-stabilized zirconia(YSZ) material directly on a carrier substrate of silicon; and whereinthe carrier substrate of silicon comprises two silicon wafers joinedtogether defining a detachable interface, the detachable interfaceincluding a joint between the two silicon wafers including one surfaceof the two silicon wafers that is roughened.
 17. The substrate of claim16, wherein the monocrystalline seed layer of YSZ material is present inthe form of a plurality of tiles.
 18. The substrate of claim 16, whereinthe detachable interface is configured to be detached by a laserdebonding technique and/or chemical attack and/or by application ofmechanical stress.